1Field of the Invention
The present invention relates to a view finder optical system for cameras. Especially, the invention relates to a compact and variable magnification Keplerian finder.
2. Related Background Art
In lens shutter cameras, it is also a trend in recent years that those equipped with a phototaking lens having a variable focal length are of the major concern. With this trend, it is required that the finders adopted for such cameras are also capable of performing a continuously variable magnification. Particularly, the Keplerian finders are on demand increasingly for those to be used for high-class cameras because with them, it is possible to divide the field of vision of the finder definitely by the use of its frame therefor and there occurs less ghost or flare. Accordingly, various proposals have been made in recent years regarding the optical system for the Keplerian finder having a variable magnification. Among them, U.S. Pat. No. 4,842,395 and Japanese Patent Laid-Open Application 1-116616 are known, for example.
However, the above-mentioned known finder optical systems still have rooms for improvements in the miniaturization and simplification of its structures. For example, in the embodiment disclosed in the U.S. Pat. No. 4,842,395, if the variation of its magnification (hereinafter referred to as zoom ratio) is as great as approximately 1.8 times, the structure becomes complicated, and if the structure remains to be simple, its zoom ratio cannot be more than 1.5 times, which is not satisfactory in providing a desirable performance. Also, in the Japanese Patent Laid-Open Application 1-116616, a finder optical system having a zoom ratio of substantially more than two times is disclosed. Nevertheless, the disclosed finder is of a considerably complicated structure and a large type, comprising an objective lens of three lens groups of negative, positive, and positive, which makes it difficult to incorporate the system in a miniaturized camera.